Many systems, including communications systems, employ variable control to adjust system parameters in order to accommodate changes in the system's environment. Such adaptive control may be employed in process flow, manufacturing, communications, or any other field in which a control parameter varies over time and adjustments are made to control variables (such as tap weights, in digital control systems) to accommodate those changes.
For example, a receiver for a ten gigabit per second (10 Gbps) optical communications system must contend with polarization mode dispersion, uncompensated chromatic dispersion, and imperfect channel filtering, all of which create inter-symbol Interference. The magnitude of the inter-symbol interference attributable to polarization mode dispersion can vary significantly, and the time scale of the variations ranges from milliseconds to hours at a time. To successfully compensate for such time-dependent inter-symbol interference, a receiver must adaptively compensate for distortions in a manner that accommodates both the magnitude of the distortion and the rate of change of the distortion. Without such compensation the receiver suffers from a power penalty and a corresponding decrease in span length. A telecommunications system employing such uncompensated receivers would be required to regenerate the communications signals at shorter intervals, with concomitant increases in fixed and recurring costs and reduced system reliability.
Electronic equalizers have been used extensively in data transmission systems to compensate for the conditions that create inter-symbol interference. Real-time adaptive equalizers are employed to compensate for time-varying distortions, such as polarization mode dispersion, to guide a receiver to convergence during, and without interruption of, the payload signal transmission. Conventional adaptive controllers used in communications systems typically require the digitization of the payload signal and/or a significant amount of signal processing at the transmission speed of the payload signal. However, because of their complexity, conventional approaches which employ, for example, zero-forcing or least-mean-square algorithms are incapable of compensating for time-varying distortions in high speed signals. That is, now, and for the foreseeable future, controllers cannot operate at sufficient speeds to employ such algorithms on signals such as 10 Gbps signals. Furthermore, even if the speed of circuitry increases sufficiently to permit an equalizer to employ such algorithms on signals operating at these speeds, the demand for operation at even higher speeds will preclude the use of such complex algorithms in future real time adaptive equalizers.
A system and method for effecting relatively simple adaptive control would be highly desirable, not only in high speed communications, but in all adaptive control systems that could take advantage of high-speed convergence of control parameter values.